The present invention generally relates to the control of fluid flow about solid surfaces and, more particularly, to fluid flow control through the use of pulse modulated actuators.
Manipulation and control of shear flow evolution has tremendous impact for influencing system performance in diverse technological applications, including lift and drag of aerodynamic surfaces, flow reattachment to wings, and aircraft stall management. The fact that shear flows are dominated by the dynamics of a hierarchy of vortical structures, evolving as a result of inherent hydrodynamic instabilities in the flow, suggests control strategies based on manipulation of these instabilities by the introduction of disturbances at the flow boundary. It is generally recognized that suitable actuators having fast dynamic response and relatively low power consumption are the foundation of any scheme for the manipulation and control of such shear flows.
One particular application for shear flow manipulation devices is related to airframes or their lifting surfaces. The shear flow that is generated by an aerodynamic lifting surface presents unique challenges, and manipulation of this shear flow will greatly affect flight performance. It is commonly understood that separation of a free stream flow about a lifting surface is generally not desired. Once the flow about a lifting surface separates, the aerodynamic surface experiences a dramatic decrease in lift force produced and a corresponding dramatic rise in the drag force generated by the aerodynamic surface.
Because of the undesirable consequences of flow separation, active manipulation of separated flows over lifting surfaces at moderate and high angles of attack has been the focus of a number of investigations since the early eighties. The goal of such active manipulation is improving the aerodynamic performance and extending an aircraft""s flight envelope by inducing complete or partial flow reattachment to the lifting surface.
Some efforts of designers to modify the flow about an aerodynamic surface, and lend to the reattachment of flow over a lifting surface, have centered on injection of momentum into the boundary layer of the flow. For example, the method disclosed by U.S. Pat. No. 4,802,642 to Mangiarotty involves the retardation of a flow""s transition to turbulence in order to improve the aerodynamic performance of a lifting surface. The Mangiarotty apparatus propagates acoustic excitation above the Tollmien-Schlichting frequency in an attempt to disrupt Tollmien-Schlichting waves as they begin to form and thereby delay the onset of turbulence. Although the Mangiarotty method changes the drag characteristic of a lifting surface, the mean velocity field and thus apparent aerodynamic shape of the surface, remains unchanged.
Such devices as slots and fluid jets have also been used to inject momentum into the boundary layer in order to prevent flow separation. Although effective at delaying flow separation, none of these devices alter the apparent aerodynamic shape or mean velocity field of a given aerodynamic surface. Additionally, the locus of the flow stagnation points remain largely unchanged.
More recently, synthetic jet actuators have been developed for the control and manipulation of shear flows. Synthetic jet actuators are described in U. S. Pat. No. 5,758,823 to Glezer et al., issued Jun. 2, 1998, which is incorporated herein by reference. As explained in the Glezer et al. patent, a synthetic jet actuator, in its most simple form, comprises a housing defining an internal chamber. An orifice is present in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within the internal chamber. As the volume of the chamber is increased, ambient fluid. is drawn into the chamber. As the volume of the chamber is decreased, the ambient fluid in the chamber is ejected such that a series of fluid vortices is generated and projected in an external environment, out from the orifice of the housing. These vortices move away from the edges of the orifice under their own self-induced velocity. As the vortices travel away from the orifice, they synthesize a jet of fluid, a xe2x80x9csynthetic jet,xe2x80x9d through entrainment of additional ambient fluid.
It has been discovered that a synthetic jet actuator may be embedded in a solid body, or surface, with the jet orifice built into the body/surface. The interaction of a free stream fluid flow about the body with a synthetic jet stream will change the overall fluid flow field around the solid body. In fact, a synthetic jet actuator operated in a lifting surface will alter the apparent aerodynamic shape of the surface. This phenomenon is fully described in U. S. Pat. No. 5,957,413 to Glezer et al., issued Sep. 28, 1999, which is hereby incorporated herein by reference.
It has been discovered that the effectiveness of synthetic jet actuators are controlled and/or limited by certain parameters. Particularly, placement and strength of these jet actuators are important. Synthetic jet actuators, when used on a wing, should be placed near the point on the wing where the flow is expected to separate. Placement away from this point will reduce the effectiveness of the jet actuator. Adding to these limitations, the jet actuator must have sufficient strength to create the needed alteration of the aerodynamic shape. An undersized synthetic jet actuator is much less effective. Additionally, the ideal strength and/or placement of a synthetic jet actuator may change with flight conditions.
Thus there exists a need in the art to improve the performance of synthetic jet actuators. There also exists a need to optimize the performance of synthetic jet actuators that are underpowered, either due to changes in flight conditions or due to inherent limitations on the system. The apparatus and method described herein, in both the text and figures, seeks to remedy the problems in the art and provide a method and apparatus for improving the performance of actuators, and particularly, synthetic jet actuators.
Briefly described, the present invention involves the use of actuators, particularly synthetic jet actuators for modification of fluid flow about the various surfaces. The present invention is primarily concerned with pulse modulating synthetic jet actuators in order to take advantage of transient responses and thereby improve synthetic jet actuator performance. However, many different types of actuators may benefit from the pulse modulation technique described herein.
One aspect of the apparatus disclosed below is a system for modifying an aerodynamic property of an aerodynamic surface in a fluid flow. The fluid flow could comprise a free stream fluid flow, or an internal fluid flow, such as in a nozzle, diffuser, or compressor. The system preferably comprises a synthetic jet actuator embedded in an aerodynamic surface. In one aspect of the preferred embodiment, the aerodynamic surface may be a wing of an aircraft. The synthetic jet actuator typically has a jet housing defining a chamber, where the chamber is in fluid communication with the fluid. This fluid communication may be accomplished via an orifice in a wall of the housing. Additionally, a portion of said housing is preferably moveable such that the volume of the chamber can be adjusted. This portion of the housing may comprise a flexible diaphragm. The system also comprises a device for changing the position of the moveable portion of the housing. This may comprise a piezoelectric actuator and a power supply. The system also comprises a controller for automatically cycling the position changing device between on and off at a predefined frequency. In this way, the synthetic jet actuator is pulse modulated in order to enhance the synthetic jet actuator""s performance.
In another aspect, the invention may been seen as a method of controlling a synthetic jet actuator. The method preferably comprises the steps of driving a synthetic jet actuator at a first frequency and turning the synthetic jet actuator on and off at a second frequency.
Other systems, methods, features. and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.